Such a method for providing a loop-free telecommunication network is generally known in the art. Therein, the telecommunication network comprises at least one Metropolitan Area Network (MAN) and/or Wide Area Network (WAN) providing Virtual Private Local Area Network (LAN) Service (VPLS), i.e. connectivity between possible Layer-2 networks as if they were connected using a LAN. Each Layer-2 network may comprise bridges and LAN segments.
In a telecommunication network of the Layer-2 type, no connectivity loops should exists. To this end, bridges run a known protocol in the Layer-2 network to keep it loop-free and disable or cut the links that cause loops.
The protocol to prevent loops is preferably the Spanning Tree Protocol (STP) or the Rapid Spanning Tree Protocol (RSTP) as defined in the standard IEEE 802.1D-2003. This protocol is running on each bridge in the network and the bridges are exchanging “bridge Protocol Data Units” (BPDUs; PDU being a general name for a data packet) to spread topology information throughout the network.
Although internal in the VPLS-network there are no effective loops, by design of the VPLS, loops in the network can be constructed by connections outside of the VPLS-network.
A problem is how to resolve possible loops that are caused by interconnecting Layer-2 networks and a VPLS-network, hereafter also merely called VPLS. The straightforward idea is to let the nodes in the VPLS run “regular STP” or “regular RSTP” on the ports of all its nodes. However, this would create possible disabling of links internal to the VPLS and in turn would render the VPLS unusable.
A first solution is proposed in a VPLS IETF-draft. It consists in making the VPLS transparent for the (R)STP protocol. This is achieved by tunneling all BPDUs through the VPLS as if they were normal data packets. However, a disadvantage of this known solution is that the operation of the (R)STP is not efficient on the links to the VPLS. As seen from the bridge connecting to the VPLS, the link is a link to a shared medium: all other bridges attached to the VPLS appear to be on the same medium. This causes (R)STP to operate significantly slower.
Another possible solution is proposed by the “Timetra Spanning Tree Protocol” (TSTP) developed by Alcatel™. In this other known solution, the nodes in the VPLS are participating in the STP protocol on its external interfaces only. This works when a dedicated, unique role is assigned to the VPLS-network: the role of “root bridge”. The TSTP solves the above-mentioned disadvantage of inefficiency. However, a new disadvantage is introduced. The VPLS claims on its external interfaces to have the root bridge, thus making it impossible to have any other bridge of a Layer-2 network connected to the VPLS to be the root bridge from which the STP protocol starts operating, as it is generally the case.